In order to avoid certain batteries being subjected to excessive discharge which might damage them by “inversion”, proposals have been made to fit such batteries with end-of-discharge control apparatus. That type of apparatus generally comprises control means serving, at the end of discharge, firstly to measure the voltage across the terminals of each module of the battery, then to compare said end-of-discharge voltages with a module threshold voltage referred to as a “stop” voltage, so as to be able to interrupt operation of the battery when at least one of said measured voltages is below its stop voltage. Such apparatus is described in particular in patent document SU-1 686 539.
That type of control apparatus is entirely suitable for batteries comprising modules each having a small number of cells, and to applications in which the current and/or temperature ranges involved are small. However, when the battery voltage becomes large, typically greater than about 20 volts (V), that type of control apparatus can become troublesome. When one of the cells in a module is short circuited, in particular because of premature wear or failure, the voltage measured across the terminals of the faulty module becomes less than the voltages measured across the terminals of the other modules. Consequently, once the control apparatus discovers that the voltage across the terminals of the faulty module is less than the stop voltage, it interrupts operation of the battery even though non-negligible usable capacity still remains in the battery.
For example, in a battery comprising three modules each comprising ten cells presenting an average voltage of 1.2 V, when one of the cells in one module is short circuited, the mean voltage of the battery is equal to 34.8 V instead of 36 V, which represents a voltage loss of about 3.3%. If such a battery is fitted with control apparatus configured for a stop voltage of 10.5 V, so as to prevent end-of-discharge inversions when battery capacity has changed to about 18% for low discharge current and to about 90% for maximum discharge current, then said apparatus will very quickly detect that the module operating on nine out of ten cells presents a voltage lower than the stop voltage, and will consequently decide to interrupt operation of the entire battery, even though it still has usable capacity lying in the range about 18% to about 90% and has lost only 3.3% of its voltage.